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Abstract

The accuracy and the resolution of water-vapor measurements by use
of the ground-based differential absorption lidar (DIAL) system of
the Max-Planck-Institute (MPI) are determined. A theoretical
analysis, intercomparisons with radiosondes, and measurements in
high-altitude clouds allow the conclusion that, with the MPI DIAL
system, water-vapor measurements with a systematic error of <5% in
the whole troposphere can be performed. Special emphasis is laid on
the outstanding daytime and nighttime performance of the DIAL system in
the lower troposphere. With a time resolution of 1 min the
statistical error varies between 0.05 g/m3 in the near
range using 75 m and—depending on the meteorological
conditions—approximately 0.25 g/m3 at 2 km using 150-m
vertical resolution. When the eddy correlation method is applied,
this accuracy and resolution are sufficient to determine water-vapor
flux profiles in the convective boundary layer with a statistical error
of <10% in each data point to approximately 1700 m. The
results have contributed to the fact that the DIAL method has finally
won recognition as an excellent tool for tropospheric research, in
particular for boundary layer research and as a calibration standard
for radiosondes and satellites.

References

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a The specifications are fulfilled in the
field for three days.2,18,19b Δνon,off, difference
between on-line and off-line laser frequencies.c Measured after the beam expander.dRl,0, distance between the
optical axes of the laser and of the telescope at ground.eRt, radius of the telescope.f FOV, half-angle field of view of the telescope.g Δνf, FWHM of the
interference filter.

a The statistical errors in the range up to
2 km are mean values using a varying height resolution of 75 m in
the near range and 150 m in the far range. The errors in DIAL
measurements can be transformed to different resolutions by using Eqs.
(7) and (8). For a Raman lidar the transformation rule is
the same for a change in time and height resolution and it applies to
Eq. (7). PA is the product of the average power of
the laser transmitter multiplied with the area of the telescope used in
the detector system.b In the presence of strong gradients of the
inverse scattering ratio it can be higher if the Doppler correction is
not sufficient.c Depends on the accuracy of the Raman lidar
calibration.d Taking into account that daytime airborne
measurements would not be sufficiently accurate.

a The specifications are fulfilled in the
field for three days.2,18,19b Δνon,off, difference
between on-line and off-line laser frequencies.c Measured after the beam expander.dRl,0, distance between the
optical axes of the laser and of the telescope at ground.eRt, radius of the telescope.f FOV, half-angle field of view of the telescope.g Δνf, FWHM of the
interference filter.

a The statistical errors in the range up to
2 km are mean values using a varying height resolution of 75 m in
the near range and 150 m in the far range. The errors in DIAL
measurements can be transformed to different resolutions by using Eqs.
(7) and (8). For a Raman lidar the transformation rule is
the same for a change in time and height resolution and it applies to
Eq. (7). PA is the product of the average power of
the laser transmitter multiplied with the area of the telescope used in
the detector system.b In the presence of strong gradients of the
inverse scattering ratio it can be higher if the Doppler correction is
not sufficient.c Depends on the accuracy of the Raman lidar
calibration.d Taking into account that daytime airborne
measurements would not be sufficiently accurate.